For generations, Glaswegians have been fortunate enough to have enjoyed a supply of naturally, lightly treated drinking water – claimed to be amongst the purest in Europe – that flows from hills and glens of the Scottish highlands. This supply, amassed in the picturesque Loch Katrine, 26 miles away in the world-famous Trossachs beauty spot, is transferred through a series of gravity-fed aqueducts to a water treatment works built 150-years ago at Milngavie, a town on the outskirts of the city.
Although the plant had been identified as in need of replacement, it took an incident involving contamination of the supply by the cryptosporidium bug in 1999 to spurn (then) West of Scotland Water to prioritise the £120 million investment. Now, under Scottish Water, the ‘Katrine Water Project’, supplies over 700,000 residents of Greater Glasgow with access to greatly improved quality water that has been upgraded to European standards.
Through this state-of-the-art plant – cut out of a hillside and hidden behind trees – the chlorinated supply from the Victorian-built works has been replaced by water from the new adjacent facility equipped with flocculating tanks and 12 rapid gravity filters to remove solids. According to Scottish Water, the completion of the new plant means that the improved water supply has not only provided a significant health gain but is contributing to the continued economic development of this area of the west of Scotland.
Ronnie Mercer, chairman of publicly-owned Scottish Water, said that “The Victorians were the first to harness this natural supply through the original Loch Katrine supply system. What we have done with our investment in the new Milngavie works is to make the water clearer and fresher. Across Scotland water quality standards are the highest they’ve ever been. Glasgow has always been blessed with a good natural supply of untreated water from Loch Katrine, acclaimed as being among the best in the world.”
“It’s of important concern to us that the Greater Glasgow area receives the very best water available. The latest filtration technology at the new plant means our customers are receiving water quality of the highest standard.”
The switch over to the new plant was an carefully planned overnight operation which saw Scottish Water engineers close down operations at the old works while the first flows of the new supply were allowed to enter the network through ten original one-metre diameter mains. Water from the new facility now serves homes and businesses across Glasgow as well as parts of East and West Dunbartonshire, East Renfrewshire, Renfrewshire and South Lanarkshire. The new Milngavie plant can deliver up to 240 million litres of water a day.
The Scottish Parliament’s Infrastructure Minister, Stewart Stevenson, commented “Households and businesses in and around Glasgow can now access drinking water that meets 21st century standards…these improvements will help to make Glasgow a healthier city. The development of this new Scottish Water facility builds on the great engineering achievements of the Victorians by ensuring that the clear waters from Loch Katrine are made even cleaner and safer for the public to drink.”
All did not go smoothly on the journey to deliver the new water treatment facility. In 2001, the original planning application was turned down following stiff resistance by local residents. However, a campaign to win the hearts and minds of local residents was more successful and since construction started in March 2004, only a small number of issues arose and all were quickly resolved.
A major problem faced by Scottish Water in designing and constructing the replacement water treatment plant was in the complex formation of tunnels, intake structures and a pumping station. The dilemma was that the existing adjacent reservoirs at Craigmaddie and Mugdock lay some 30 metres below the level of the new, replacement facility. Furthermore, this engineering challenge had to be solved without the operation of the former treatment works remained uninterrupted.
Four 400kW pumps were installed in each of the two intake tunnels to lift water up to the plant. The pumps have structures that are constructed within the twin reservoirs – inside the embankments and surrounding Victorian infrastructure which are all of Listed building status.
Water is drawn through the pumping station from both reservoirs and is delivered via a 1100mm diameter steel pipe to the new treatment works 300 metres away.
Scottish Water’s Katrine Water Project manager, Gus Watt, said: “Due to the varying nature of the ground – dolerite, sandstone and boulder clay – the 1600mm diameter tunnels were excavated manually using a handshield method. “Scottish Water, contractor, sub-contractor and designers worked closely together to deliver this technically-difficult element of construction within very demanding constraints.”
The main focus of the construction at Milngavie may have been the provision of the replacement water treatment works and two reservoirs to store treated water, however, the Katrine Project’s accompanying schemes include the £2 million Strathblane Cross Connection. This is able to transfer up to 360 million litres of water a day from the Katrine aqueducts to the Lomond aqueducts that are separated vertically by a 30-metre drop, by natural gravity flow. This development is saving customers more than £500,000 a year in electricity pumping charges. It also includes the link-up of a £5 million pipeline from the nearby Balmore Water Treatment Works to the main that runs from Craigmaddie Reservoir.
The lead contractor (formerly MJ Gleeson), now Black & Veatch, was commissioned to construct and commission a service reservoir, clearwater tank, transfer systems and the new treatment works, all operational within three years. Commenting on this massive contract, Black & Veatch’s Senior Contracts Manager, Mark Allan, said: “From the outset it was appreciated that developing the new works within the residential and highly-sensitive Milngavie area would require careful planning, co-ordination and consultation with the local community. One of our key objectives was to minimise the impact of construction activities on the local area.’’
Within the Katrine Project, 60% of the permanent works are located underground and purposely designed to hug the curve of a hillside in order to blend with the surroundings and not break the existing perimeter treeline. Following a year-long trial in a pilot plant at Milngavie, the choice of process was decided. The raw water supply is chemically treated by dosing lime for Ph correction and polyelectrolite for coagulation prior to entering flocculation tanks.
There is a wastewater recycling facility incorporated within the plant where sludge is separated through lamellas. Sludge is discharged to sewer and the supernatant is pumped to the head of the works. Disinfection is by a sodium hypochloride solution, an improved safety method when compared to the former use of chlorine gas. Once processed, treated water passes to a 80 million litre, high-level, clearwater storage tank that, initially, feeds the high level supply zone within Glasgow. It also delivers to another similar-sized storage tank – the service reservoir – that supplies a low level zone mainly within the centre of Glasgow.
Complexities of construction of the 125-metre-long service reservoir, in reinforced concrete, were principally associated with the size and tight construction programme. Gus Watt added: “Involvement and interaction with the local community have always been high on the agenda of the Katrine Water Project team whose formation led to the creation of a community forum and a two-way flow of information aimed at addressing public concerns regarding the project. It also helped promote an understanding of construction issues and requirements”
Later a separate steering group was established to examine possible post-construction conservation and recreational activities at the reservoir complex. The main objective of these combined activities was to ensure, according to Watt, that “the particular ambience of the area was maintained as much as possible during the construction period. We are delighted that, when we leave, we will do so having ensured that the minimum amount of disturbance was caused and the reservoirs complex is as close to what it was when we came.”
Scottish Water’s current investment programme to 2010 will amount to £2.4 billion. The programme will see Scottish households enjoy improved drinking water; take action against leaks and provide a better protected river and marine environment.
Project facts
The Katrine Water Project represents a £120 million investment – the biggest water project in Scotland and one of the largest water treatment schemes in Europe.
• Over 700,000 residents of Greater Glasgow now receive high quality drinking water from Milngavie.
• The new plant is capable of delivering up to 240 million litres of water a day.
• The two cavernous reservoirs at the Milngavie complex each contain 20 million gallons.
• Over 4,400 kilometres of reinforcing steel bars have been used – sufficient to stretch from Glasgow to Halifax, Nova Scotia.
• The footprint of the treatment works measures 8,600 m2.
• The total amount of concrete used amounted to around 110,000 tonnes.
• Excavated material from Barrachan has been used to cover the Bankell reservoir for landscaping •
Filtration technical description
The new Milngavie WTW is designed for a maximum plant outlet flow of 240 MLD.
Loch Katrine raw water is treated through a direct filtration plant, utilising coagulation and dual media filtration.
Raw water conditioning
The untreated water is dosed with limewater to increase the pH before coagulant dosing. The combined raw and recycle water is dosed with a 0.15% w/w solution of limewater in the inlet channel. The limewater dosing is controlled by flow pacing from the plant flow, trimmed by a feedback from the downstream pH meter to ensure that close control of the pH is maintained. A limewater solution has been used instead of a slurry as maintaining accurate flocculation pH control is crucial to the process due to the low buffering capacity of the raw water. It also has the benefit that carrier water isn’t required and the dosing lines aren’t as prone to blockages.
Coagulation
The conditioned stream is then dosed with coagulant in the single inlet channel. The coagulant used is Aluminium Sulphate, at an average dose of 0.8 mg/l as Al.
Flocculation
It may be required to dose flocculation aid polymer during high colour events (>30 Hazen). There is a facility to dose with polyDADMAC. The dosing point for the flocculation aid polymer will be the same as the coagulant, into the single inlet channel. The coagulated water then flows by gravity to two flocculation streams, which have a combined residence time of 5 minutes at the instantaneous maximum flow. In each stream the water is mixed in three-stage flocculation tanks, which are separated by perforated plate baffles.
The flocculated water flows to where the two streams combine and is then dosed with a filtration aid polymer, (polyacrylamide) at an average dose of 0.08 mg/l, via a static mixer in the channel prior to the filters.
Filtration
The plant has 12 dual media rapid gravity filters, each containing a 600 mm layer of normal cut anthracite (No.2) over 600 mm of 14/25 sand. Each filter is monitored for flow, filtered water turbidity, head loss across the filter media and flow normalised head loss across the filter media. All of these are trended on the SCADA system.
Washwater recovery
In normal operation the filters would be expected to be called to wash on time elapsed and each filter will be washed once a day. Under these circumstances only one filter will need to be out for washing at any one time, though it is possible to carry out a maximum of 24 filter washes in a single day.
The filter backwash consists of an air scour followed by a combined air/water wash; to ensure that the dirt is removed from the media, followed by a high rate re-grade wash. After washing is complete the filter is refilled and the filter run to waste sequence will commence. The filter will be run to waste to the Balance Tank until the filtered turbidity falls below a pre-set value (<0.1 NTU), to allow the filter to ripen and minimise the risk of cryptosporidium going into supply.
Filter backwash can be initiated in four ways
1) Manual demand for backwash from Operator
2) High filter outlet turbidity (> 0.1 NTU)
3) High filter flow normalised head loss
4) Time elapsed since last backwash
The dirty washwater passes to the washwater holding tanks that feed lamella clarifiers to achieve solid liquid separation. The clarifiers are run in parallel and operated with a continuous inflow. However, redundancy is included in the system as under normal back-washing conditions the total volume of washwater produced can be treated by one clarifier stream.
Supernatant from the lamellas returns to the Balancing Tank for recycle to the raw water pumping station and sludge passes to the sludge holding tank, from where it is discharged to sewer. The lamellas are designed to provide a sludge with a solids content, that will be a minimum of 0.8% and a maximum of 2 % w/w.
Supernatant with turbidity above 10 NTU is not returned to the balance tank, due to the increased risk of cryptosporidium passing into supply. The lamellas have been designed to achieve turbidity below 2 NTU as a 95%ile at maximum flow and solids loading and a turbidity below 5 NTU as a maximum. The washwater supernatant recycle will vary between 2.5 and 5.0 % of the total plant flow in accordance with the Bouchier recommendations.
The dirty washwater is pumped and dosed with polyacrylamide before entering flocculation tanks, to improve separation in the lamellas. The lamellas consist of a series of inclined metal plates, where the clarified liquid flows upward and over weirs into the balancing tank while the solids settle on the inclined parallel plates and slide onto the floor of the tank. The scrapper mechanism on the tank floor then moves the sludge into collection hoppers, which are desludged on a adjustable time interval into the sludge holding tank. lamellas have been chosen for mingavie due to their small foot print area in comparison to a conventional settlement tank, and product quality.
Orthophosphoric acid dosing
Orthophosphoric acid for plumbosolvency control is dosed downstream of the filters at the same point as the sodium hypochlorite addition.
Disinfection
10% Sodium hypochlorite solution is used as the disinfectant. This is delivered to the site as a 15% solution and diluted on site to 10%. The hypochlorite dose is flow paced with a feedback loop trim from the post dosing chlorine residual sample, to give closed loop control of the chlorine dose. Hypochlorite has been chosen as a safer alternative to chlorine gas.
Treated water stabilisation
The pH of the disinfected water is raised by the addition of limewater (0.15% w/w).
Final water storage
There are two treated water storage tanks, the Clearwater Tank and the Service Reservoir. The Clearwater Tank directly feeds the high level supply zone within Glasgow. It also feeds the Service Reservoir that supplies the low level supply zone mainly within the centre of Glasgow.
Contact:
Norrie Hunter
Email: norriehmedia@yahoo.com